This invention relates to near infrared analysis instruments and more particularly to instruments of rugged design for field use for analysis of particulate material.
A powerful technology for analyzing or identifying material involves measuring the reflectance from or the transmission through the material to be analyzed at narrow wavelength bands in the near infrared range, known as NIR. A typical analyzing instrument comprises a spectrophotometer employing a grating to disperse the infrared light into a spectrum to enable the absorption of the matter to be measured at narrow wavelength bands. In one type of instrument, broad band NIR is dispersed into a spectrum by an oscillating grating. As the grating oscillates, the wavelength of the dispersed light passing through an exit slit is scanned through the spectrum. The infrared light passing though the exit slit irradiates a sample of material to be analyzed. The light transmitted through the sample or reflected from the sample is detected to obtain a measurement of the absorption spectrum of the sample. Alternatively the broad band NIR may irradiate the sample, in which case the light reflected from the sample or transmitted through the sample is directed to the grating to be dispersed to obtain the absorption spectrum measurements. In another type of spectrophotometer, a fixed grating is used to disperse the infrared light into a spectrum, which is detected by an array of photodetectors each positioned and shaped to detect a narrow band of the dispersed spectrum. In this kind of instrument, the sample is irradiated with broad band NIR and the light transmitted through or reflected from the sample is dispersed by the fixed grating into the spectrum detected by the array of photodetectors.
An instrument with a fixed grating array would be preferred for field use because it is more rugged than an instrument employing an oscillating grating and also it is less expensive than an oscillating grating instrument. In an instrument with a fixed grating, it is preferable to use silicon photodetectors in the detector array in order to achieve the desired resolution in the detection. However, silicon detectors are effective to detect light only below 1100 nm and accordingly the array silicon photodetectors should be arranged to detect this portion of the spectrum dispersed by the fixed grating.
When a conventional fiber-optic probe such as that shown in U.S. Pat. No. 5,166,756 or that shown in U.S. Pat. No. 5,351,322 is used to apply infrared light to a particulate sample and receive light reflected back from the sample in the range of 500 nm to 1100 nm, an excessive amount of spectrally reflected light will be received by the optic-fibers of the probe arranged to received the reflected light. This excessive spectrally reflected light travels from the transmitting fibers to the receiving fibers at or near the interface between the probe and the sample. The transmission of this excessive spectrally reflected light from the transmitting fibers to the receiving fibers is called the light pipe effect. The excessive spectrally reflected light in the 500 to 1100 nm range washes out or overwhelms the received light diffusely reflected from the sample and prevents accurate measurements of the absorption of the sample.
This problem may be overcome by using a probe which transmits the light through a sample instead of detecting the light reflected from the sample wherein the sample is made sufficiently thick that all of the light passing through the sample constitutes diffusely reflected forward scattered light. However, a probe designed to measure light transmitted through a particulate sample is much more difficult to use in the field than a probe designed to measure reflected light because a sample holder must be carefully filed with a sample for each measurement to ensure that the sample through which the light is transmitted is of a consistent density for each measurement and so that no air pockets exist in the sample. In contrast, it is much easier to use a reflectance measuring probe which can be simply inserted into the sample and reflectance measurements taken.
Accordingly, there is a need for an infrared analysis instrument with a fixed grating which can make measurements from a particulate sample by a simple insertion of the probe into the sample without the measurements being adversely effected by spectrally reflected light.
In accordance with the invention, the problems described above are overcome by using a probe in which the central fibers of the probe are used to transmit light into the sample and a ring of receiving fibers surrounding the central transmitting fibers receive light reflected back from the sample. In accordance with the invention, the distal ends of the ring of receiving fibers are set back from the distal ends of the transmitting fibers so that light transmitted into the sample from the transmitting fibers and reflected by the sample must travel through a substantial amount of the sample to get to the ring of receiving fibers. As a result, all the spectrally reflected light will be attenuated to zero and all of the light received by the receiving fibers will be diffusely reflected light, which will be attenuated at each wavelength in accordance with the absorbance of the sample.
In the instrument of the invention, the light received by the ring of receiving optic fibers is transmitted through an entrance slit of a housing to irradiate a fixed grating contained by the housing. The grating disperses the received light into a spectrum and an array of silicon photodetectors are positioned to detect narrow wavelength bands of the spectrum in the range of 500 to 1100 nm. From the signals generated by the silicon photodetectors, the material of the particulate sample can be precisely identified or accurately analyzed by mathematical processing.
Because of the design of the probe, which can be simply inserted into the sample to make a measurement, the received light analyzed by the instrument is devoid of spectrally reflected light and an accurate analysis or identification of the particulate material can be made. Because the instrument employs a fixed grating to disperse the light, the instrument is rugged and is relatively inexpensive.